Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus is disclosed including a liquid supply system configured to at least partly fill a space between the projection system and the substrate with a liquid, an outlet configured to remove a mixture of liquid and gas passing through a gap between a liquid confinement structure of the liquid supply system and the substrate, and an evacuation system configured to draw the mixture through the outlet, the evacuation system having a separator tank arranged to separate liquid from gas in the mixture and a separator tank pressure controller, connected to a non-liquid-filled region of the separator tank, configured to maintain a stable pressure within the non-liquid-filled region.

This application is a continuation of U.S. patent application Ser. No.15/251,915, filed on Aug. 30, 2016, which is a continuation of U.S.patent application Ser. No. 14/586,518, filed on Dec. 30, 2014, now U.S.Pat. No. 9,436,097, which is a continuation of U.S. patent applicationSer. No. 13/167,314, filed on Jun. 23, 2011, now U.S. Pat. No.8,934,082, which is a continuation of U.S. patent application Ser. No.12/078,635 filed on Apr. 2, 2008, now U.S. Pat. No. 8,004,652, which isa continuation of U.S. patent application Ser. No. 10/966,108 filed onOct. 18, 2004, now U.S. Pat. No. 7,379,155, each of the foregoingapplications is incorporated herein in its entirety by reference.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate (the substrategenerally has a larger surface area than the final element of theprojection system). One way which has been proposed to arrange for thisis disclosed in PCT patent application WO 99/49504, hereby incorporatedin its entirety by reference. As illustrated in FIGS. 2 and 3, liquid issupplied by at least one inlet IN onto the substrate, preferably alongthe direction of movement of the substrate relative to the finalelement, and is removed by at least one outlet OUT after having passedunder the projection system. That is, as the substrate is scannedbeneath the element in a −X direction, liquid is supplied at the +X sideof the element and taken up at the −X side. FIG. 2 shows the arrangementschematically in which liquid is supplied via inlet IN and is taken upon the other side of the element by outlet OUT which is connected to alow pressure source. In the illustration of FIG. 2 the liquid issupplied along the direction of movement of the substrate relative tothe final element, though this does not need to be the case. Variousorientations and numbers of in- and out-lets positioned around the finalelement are possible, one example is illustrated in FIG. 3 in which foursets of an inlet with an outlet on either side are provided in a regularpattern around the final element.

SUMMARY

In a liquid supply system of lithographic apparatus, a mixture of liquidand gas may be extracted during operation of the liquid supply system.For example, one or more outlets in the liquid supply system of FIGS. 2and 3 may extract a mixture of liquid and gas during exposure of thesubstrate. In another example, as discussed below in relation to FIG. 5,liquid and gas may be extracted to seal a gap between a liquidconfinement structure and the substrate during exposure of thesubstrate. A disturbance in the flow of liquid and gas in these systemsand in surrounding regions may negatively affect the imaging quality ofthe lithographic apparatus. Where a combination of liquid and gas isinvolved, there may be an issue of providing stable and reliableextraction due to the difficult flow properties of a liquid and gasmixture. Reliability may be a concern for example where damage to thelithography apparatus can occur following a failure in the liquid supplysystem or where an outage in the liquid supply system may cause delaysin production.

Accordingly, it would be advantageous, for example, to provide animproved system and method for evacuating mixtures of liquid and gasfrom components in a lithographic apparatus.

According to an aspect of the invention, there is provided alithographic apparatus comprising:

-   -   an illuminator configured to condition a radiation beam;    -   a support constructed to hold a patterning device, the        patterning device configured to impart the radiation beam with a        pattern in its cross-section to form a patterned radiation beam;    -   a substrate table constructed to hold a substrate;    -   a projection system configured to project the patterned        radiation beam onto a target portion of the substrate;    -   a liquid supply system configured to at least partly fill a        space between the projection system and the substrate with a        liquid, the liquid supply system comprising a liquid confinement        structure configured to at least partly confine the liquid        within the space;    -   an outlet configured to remove a mixture of liquid and gas        passing through a gap between the liquid confinement structure        and the substrate; and    -   an evacuation system configured to draw the mixture through the        outlet, the evacuation system comprising a separator tank        arranged to separate liquid from gas in the mixture and a        separator tank pressure controller, connected to a        non-liquid-filled region of the separator tank, configured to        maintain a stable pressure within the non-liquid-filled region.

According to a further aspect, there is provided an apparatus,comprising:

-   -   a pressurized gas input configured to provide gas under pressure        to an interface region of a container from which liquid may        escape;    -   a stabilized evacuation system configured to provide controlled        removal of a mixture of liquid and gas from the region, the flow        of gas caused by the pressurized gas input coupled with the        stabilized evacuation system being configured to limit the        escape of liquid from the container through the interface        region, the stabilized evacuation system comprising a separator        tank arranged to separate liquid from gas in the mixture and a        separator tank pressure controller, connected to a        non-liquid-filled region of the separator tank, configured to        maintain a stable pressure within the non-liquid-filled region.

According to a further aspect, there is provided a lithographicapparatus, comprising:

-   -   an illuminator configured to condition a radiation beam;    -   a support constructed to hold a patterning device, the        patterning device configured to impart the radiation beam with a        pattern in its cross-section to form a patterned radiation beam;    -   a substrate table constructed to hold a substrate;    -   a projection system configured to project the patterned        radiation beam onto a target portion of the substrate;    -   a liquid supply system configured to at least partly fill a        space between the projection system and the substrate with a        liquid, the liquid supply system comprising a liquid confinement        structure configured to at least partly confine the liquid        within the space;    -   an outlet configured to remove a mixture of liquid and gas        passing through a gap between the liquid confinement structure        and the substrate; and    -   an evacuation system configured to draw the mixture through the        outlet, the evacuation system comprising a two-phase compatible        pump and a liquid/gas homogenizer arranged between the gap and        the two-phase compatible pump, the liquid/gas homogenizer being        arranged to provide a uniform mixture of liquid and gas to the        two-phase compatible pump.

According to a further aspect, there is provided a lithographicapparatus, comprising:

-   -   an illuminator configured to condition a radiation beam;    -   a support constructed to hold a patterning device, the        patterning device configured to impart the radiation beam with a        pattern in its cross-section to form a patterned radiation beam;    -   a substrate table constructed to hold a substrate;    -   a projection system configured to project the patterned        radiation beam onto a target portion of the substrate;    -   a liquid supply system configured to at least partly fill a        space between the projection system and the substrate with a        liquid, the liquid supply system comprising a liquid confinement        structure configured to at least partly confine the liquid        within the space;    -   an outlet configured to remove a mixture of liquid and gas        passing through a gap between the liquid confinement structure        and the substrate; and    -   an evacuation system configured to draw the mixture through the        outlet, the evacuation system, comprising:    -   a main pumping line connected to a two-phase compatible pump        configured to pump the mixture,    -   a backup line connectable to a shared vacuum facility configured        to pump gas only and arranged to backup the two-phase compatible        pump, the two-phase compatible pump being configurable to        provide a deeper vacuum than the shared vacuum facility, and    -   a two-phase-compatible pressure regulator connected to the main        pumping line and the backup line.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts another liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts a further liquid supply system, comprising an immersionliquid reservoir with a liquid confinement structure and a gas seal, foruse in a lithographic projection apparatus;

FIG. 6 depicts an evacuation system according to an embodiment of theinvention, the system comprising a separator tank, a limited flowconnection and a pressure equalization connection;

FIG. 7 depicts an evacuation system according to an embodiment of theinvention, the system comprising a separator tank only;

FIG. 8 depicts an evacuation system according to an embodiment of theinvention, the system comprising a liquid/gas homogenizer and a holdingtank arranged to be pumped down by a two-phase compatible pump backed upby a shared vacuum facility;

FIG. 9 depicts an evacuation system according to an embodiment of theinvention, the system comprising a separator tank arranged to be pumpeddown by a two-phase compatible pump backed up by a shared vacuumfacility;

FIG. 10 depicts an evacuation system according to an embodiment of theinvention, the system comprising a separator tank and a purge tank, theseparator tank being arranged to be pumped down by a two-phasecompatible pump backed up by a shared vacuum facility, and the separatortank and purge tank being connected together by a limited flowconnection;

FIG. 11 depicts an evacuation system according to that shown in FIG. 10but further comprising a pressure equalization connection between theseparator and purge tanks;

FIG. 12 depicts an evacuation system according to that shown in FIG. 10but further comprising a connection between the purge tank and thetwo-phase compatible pump and shared vacuum facility;

FIG. 13 depicts an evacuation system according to an embodiment of theinvention, the system comprising a separator tank only with a rotatingwheel liquid pump arranged to drain the liquid from the separator tank;

FIG. 14 depicts an evacuation system such as that shown in FIG. 13,except that a gas jet pump is arranged to remove liquid from theseparator tank;

FIG. 15 depicts an evacuation system according to an embodiment of theinvention, the system comprising a separator tank only with a two-phasecompatible pump being configured to drain liquid from the separator tankand maintain a stable pressure in the separator tank, and with a checkvalve positioned between the pumping line configured to drain theseparator tank and the pumping line configured to maintain the pressurein the non-liquid-filled region of the separator tank;

FIG. 16 depicts an evacuation system according to that shown in FIG. 15without the check valve; and

FIG. 17 depicts an evacuation system according to an embodiment of theinvention, the system comprising a two-phase compatible pump backed upby a shared vacuum facility and controlled by a two-phase compatiblevacuum pressure regulator.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam PB (e.g. UV radiation or DUV radiation).    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PL configured to project a pattern imparted to the radiation        beam PB by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as a-outer anda-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam PB is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam PB passes through the projection system PL, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam PB. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam PB, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

-   -   1. In step mode, the mask table MT and the substrate table WT        are kept essentially stationary, while an entire pattern        imparted to the radiation beam is projected onto a target        portion C at one time (i.e. a single static exposure). The        substrate table WT is then shifted in the X and/or Y direction        so that a different target portion C can be exposed. In step        mode, the maximum size of the exposure field limits the size of        the target portion C imaged in a single static exposure.    -   2. In scan mode, the mask table MT and the substrate table WT        are scanned synchronously while a pattern imparted to the        radiation beam is projected onto a target portion C (Le. a        single dynamic exposure). The velocity and direction of the        substrate table WT relative to the mask table MT may be        determined by the (de-)magnification and image reversal        characteristics of the projection system PL. In scan mode, the        maximum size of the exposure field limits the width (in the        non-scanning direction) of the target portion in a single        dynamic exposure, whereas the length of the scanning motion        determines the height (in the scanning direction) of the target        portion.    -   3. In another mode, the mask table MT is kept essentially        stationary holding a programmable patterning device, and the        substrate table WT is moved or scanned while a pattern imparted        to the radiation beam is projected onto a target portion C. In        this mode, generally a pulsed radiation source is employed and        the programmable patterning device is updated as required after        each movement of the substrate table WT or in between successive        radiation pulses during a scan. This mode of operation can be        readily applied to maskless lithography that utilizes        programmable patterning device, such as a programmable mirror        array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

Another immersion lithography solution with a localized liquid supplysystem solution which has been proposed is to provide the liquid supplysystem with a liquid confinement structure which extends along at leasta part of a boundary of the space between the final element of theprojection system and the substrate table. Such a system is shown inFIG. 5. The liquid confinement structure is substantially stationaryrelative to the projection system in the XY plane though there may besome relative movement in the Z direction (in the direction of theoptical axis). A seal is formed between the liquid confinement structureand the surface of the substrate. In an embodiment, the seal is acontactless seal such as a gas seal. Such a system with a gas seal isdisclosed in U.S. patent application Ser. No. 10/705,783, herebyincorporated in its entirety by reference.

FIG. 5 depicts an arrangement of a reservoir 10, which forms acontactless seal to the substrate around the image field of theprojection system so that liquid is confined to fill a space between thesubstrate surface and the final element of the projection system. Aliquid confinement structure 12 positioned below and surrounding thefinal element of the projection system PL forms the reservoir. Liquid isbrought into the space below the projection system and within the liquidconfinement structure 12. The liquid confinement structure 12 extends alittle above the final element of the projection system and the liquidlevel rises above the final element so that a buffer of liquid isprovided. The liquid confinement structure 12 has an inner peripherythat at the upper end preferably closely conforms to the shape of theprojection system or the final element thereof and may, e.g., be round.At the bottom, the inner periphery closely conforms to the shape of theimage field, e.g., rectangular though this need not be the case.

The liquid is confined in the reservoir by a gas seal 16 between thebottom of the liquid confinement structure 12 and the surface of thesubstrate W. The gas seal is formed by gas, e.g. air, synthetic air, N₂or an inert gas, provided under pressure via inlet 15 to the gap betweenliquid confinement structure 12 and substrate and extracted via anoutlet to an evacuation system hose 14. The overpressure on the gasinlet 15, vacuum level on the outlet of the hose 14 and geometry of thegap are arranged so that there is a high-velocity gas flow inwards thatconfines the liquid. It will be understood by the person skilled in theart that other types of seal could be used to contain the liquid.

FIG. 6 shows an evacuation system 30 according to an embodiment of theinvention. The evacuation system 30 provides the driving force for theremoval of a mixture of liquid and gas from the interface region 28between the liquid confinement structure 12 and the substrate W via theevacuation system hose 14. The operation of the gas seal and its abilityto carry out its function without excessive disturbance to the liquidconfinement structure 12, substrate W or immersion liquid, may depend onthe quality of the gas flow in the seal 16 and therefore on the pumpingperformance of the evacuation system 30. According to the embodimentshown in FIG. 6 and further embodiments discussed below, a suitablystable and reliable performance may be provided through the use of aseparator tank 35.

One way in which reliability may be achieved according to this approachis that a system based on a separator tank concept may be conceived witha minimal number of components. Additionally or alternatively, whereembodiments are directed to use in a lithographic apparatus, forinstance, it may be possible to make effective use of systems alreadyavailable in such apparatus, such as a shared vacuum facility and a highpressure gas source, thus possibly minimizing costs and optimizing useof space. An additional or alternative advantage of a system based on aseparator tank principle is that, for a broad range of viscosities, itsoperation may not be very dependent on the properties of the liquidbeing pumped. This could be in contrast to more complex liquid/gaspumping systems, which may be highly dependent on the flow properties ofthe mixture.

Additionally or alternatively, the use of a separator tank as a means toseparate liquid and gaseous phases may offer an advantage from a noiseperspective. Passive separation, as occurs in the separator tank underthe influence of gravity, may reduce vibrational noise and heating,which might otherwise be transmitted through to the substrate or otherimaging-critical element in such a way as to degrade the performance ofthe lithographic apparatus and/or transmitted to the environment aroundthe apparatus so as to have a negative impact on that environment.

Additionally or alternatively, the internal components of the separatortank, and related components (such as those dedicated to draining theseparator tank), rely on valves rather than pumps and so may confer anadvantage in terms of reliability over liquid/gas processing systemsthat are based directly on pumps.

Other or alternative advantages may be discussed below in the context ofone or more particular embodiments but should be understood as beingmore generally applicable where appropriate. In addition, although oneor more embodiments are discussed in terms of liquid/gas mixturesremoved from a gas seal in a lithographic apparatus, it should beunderstood that one or more embodiments of the invention may beapplicable to other systems, lithographic or non-lithographic, involvingremoving a mixture of liquid and gas in a controlled manner. Such othersystems may include, for example, the removal of a liquid/gas mixturefrom underneath the substrate during imaging in a lithographicapparatus.

According to an operating mode of the embodiment depicted in FIG. 6, themixture of liquid and gas is drawn from the interface region 28 viaevacuation system hose 14 to an opening in the upper region of theseparator tank 35. The driving force for this movement of liquid and gasis a low pressure maintained in the separator tank 35. Liquid settlestowards the bottom of the tank and gases are pushed to the top of thetank where they are largely removed by the pumping action of a separatortank pressure controller 40, which is configured to maintain a lowpressure in the separator tank 35. Hose 31 links the separator tankpressure controller to the separator tank 35 via an opening in an uppersurface 33. Positioning this opening near the upper surface 33 of theseparator tank 35 helps prevent liquid making its way into the separatortank pressure controller 40. This arrangement can ensure that theseparator tank pressure controller 40 and any pumping device associatedtherewith does not require a facility to deal with liquid, which maygreatly simplify the task of providing a stable low pressure environmentin the separator tank 35. The risk of system failure or unpredictableperformance may be greatly reduced by protecting the separator tankpressure controller 40 from liquid in this way.

Positioning the link to the separator tank pressure controller 40 in anupper surface 33 of the separator tank 35 may help ensure that theliquid and gas separation process can proceed in an efficient manner.This separation process may also be improved by taking one or more stepsto control (reduce) the flow rate of the liquid/gas mixture as it entersthe separator tank, of gas as it is pumped away by the separator tankpressure controller 40, and/or of liquid as it is drained away into apurge tank 50. This may be achieved by providing large diameter hoses(which, in particular, may be arranged to increase in diameter near tothe point of entry to the separator tank 35) and hose connections forrespective links to and from the separator tank 35.

To allow for continuous operation, the separator tank 35 should bedrained. In general, the separator tank 35 should not be allowed to fillexcessively as a buffer volume is used to damp pressure fluctuations.The buffering/damping action decreases as the size of the buffer volumedecreases because pressure fluctuations are only effectively damped bythe gas content of the separator tank 35, the liquid content beinghighly incompressible. A liquid-filled separator tank 35 will providelittle, if any, damping.

If the performance of the gas seal is not to be compromised, thedraining of the separator tank 35 should avoid disturbing the separatortank pressure. Several arrangements may be used for this purpose. FIG. 6depicts an embodiment using a purge tank 50, situated lower than theseparator tank 35 and connected via an opening in its top surface 65 toan opening in the lower surface 55 of the separator tank 35. The actualheight difference between the purge tank 50 and the separator tank 35 isnot critical as long as there is some difference, so that gravity can beused to push liquid between the tanks 35 and 50. The connection betweenthe tanks 35 and 50 can be controlled by a purge valve 70, which may beoperable to be either open or closed or be arranged to let liquid flowonly in one direction (i.e. by using a “check valve”).

According to an embodiment, a pressure equalization connection 75 may beprovided between an upper region of the purge tank 50 and an upperregion of the separator tank 35, controlled by a pressure equalizationvalve 80. In normal operation, the purge valve 70 will be left open andliquid will drain from the separator tank 35 into the purge tank 50. Thevolume occupied by liquid, and therefore the remaining volume in each ofthe tanks may change, which will affect the pressures in the tanks. Forexample, in the case where the liquid level is going down in theseparator tank 35 and up in the purge tank 50, the pressure will tend todecrease in the separator tank 35 and increase in the purge tank 50. Thechange in pressure in the separator tank 35 may compromise the stabilityof the low pressure provided by the separator tank pressure controller40, which in turn may negatively affect the performance of the gas seal.On the other hand, the increase in pressure in the purge tank 50 may actas a cushion to liquid entering from the separator tank 35 and reducethe efficiency with which the separator tank 35 is drained. The pressureequalization connection 75 may be provided to equalize the pressures inthe two tanks 35 and 50 and thus largely eliminate the above possibleconcerns. Alternatively, a connection may be provided to maintain afixed pressure difference between the separator tank 35 and the purgetank 50. By establishing a slightly higher working pressure in the purgetank 50, for example, it may require less time to pump down the purgetank 50 after an emptying procedure (see below for a furtherdescription). A balance should be struck between the time saved duringsuch a pump down phase and any reduction in the efficiency of drainingof the separator tank caused by a higher pressure in the purge tank 50.As a further variation, the same separator tank may be used as part ofan evacuation system for a plurality of sources of liquid/gas mixtures.Alternatively or additionally, a number of separator tanks 35 (perhapsmaintained at different pressures) may be connected to the same purgetank 50. In this arrangement, where the separator tanks 35 are atdifferent pressures, it may be convenient to choose to maintain thepurge tank at an intermediate pressure. Additional valves may beincluded to isolate one or more of the purge tanks from each other inorder to eliminate cross-talk between the tanks. This arrangement mayreduce or remove the need for an intermediate pressure being maintainedin the purge tank 50.

The purge tank 50 may be emptied to a liquid sink 90 via a liquid sinkvalve 95. The liquid sink 90 may be a drain or be connected to a liquidrecycling system. Emptying of the purge tank 50 may be initiated whenthe liquid level in the tank exceeds a predetermined threshold value.This may in turn be determined based on timing (i.e. a controller may beprogrammed to initiate a purge tank emptying procedure after a giventime has elapsed after the completion of a previous emptying procedure,the given time being selected by reference to calibration measurementscarried out to determine a purge tank fill-rate under standardconditions), or a liquid level sensor 52 may be provided that isconfigured to measure the liquid level and report to a controller whenthe liquid level reaches a threshold level. The liquid level sensor 52may be a kind of float sensor, for example. The option of using acontroller in conjunction with a sensor can provide for flexibleoperation, allowing the apparatus to adapt seamlessly to a change inoperating conditions, such as a variation in the rate of liquid flowinginto the evacuation system. On the other hand, the option of controllingthe evacuation system solely on the basis of a timed cycle makes itpossible to reduce the number of sensors (only emergency sensors mayneed to be included) and omit expensive control circuitry. By reducingthe number of critical components in this way, it may be possible tokeep costs low while achieving sufficient reliability. Where the liquidflow rate is variable and it is desired to use a timed cyclearrangement, the timed cycle can be set to allow for a maximum flowrate, all flow rates less than this being provided for automatically.

Emptying the purge tank 50 may be carried out by closing any gasconnections between the separator tank 35 and the purge tank 50 (via thepressure equalization valve 80 for example where it is provided),closing the purge valve 70, and then opening the liquid sink valve 95. Ahigh pressure gas source 100 may be connected to the upper region of thepurge tank 50 in order to establish a high gas pressure in this regionand force liquid more quickly from the purge tank 50 into the liquidsink 90. The pressure in this region may be controlled via valve 101.However, if the speed of draining of the purge tank 50 is not criticalthen the high pressure gas source 100 may be omitted. Once the purgetank has been emptied, the liquid sink valve 95 may be closed again.However, resuming normal operation immediately by opening the pressureequalization connection 75 would normally cause an excessively large gasflow from the purge tank 50 to the separator tank 35, which may causepressure fluctuations in the separator tank 35. This gas flow typicallyarises due to the pressure difference between the two tanks 35 and 50(which may be exacerbated in the case that the purge tank 50 is largerthan the separator tank 35). Providing a limited flow connection 145,comprising a flow restriction device 150, between the two tanks 35 and50, may reduce the size of the gas flow. This restricted connectionallows the purge tank 50 to be gradually pumped down to the samepressure as the separator tank 35 without unduly loading the separatortank pressure controller 40 by a sudden influx of gas. The limited flowconnection 145 may be provided as a separate connection (as depicted inFIG. 6) or may alternatively be implemented via the pressureequalization connection 75 (for example, by providing a controllablevalve 80 that is capable of providing a low flow impedance for normaloperation (when the purge tank 50 is already pumped down), and a highflow impedance during a phase where the purge tank 50 is being graduallypumped down after an emptying procedure). In applications where the flowrestriction device 150 may be set to have a relatively low flowimpedance, it may be possible to dispense with the pressure equalizationconnection 75 altogether and use the flow restriction device 150 as ameans to gradually pump down the purge tank 50 and to ease the flow ofliquid between the two tanks 35 and 50. The flow restriction device 150may be, for example, a needle valve, orifice or capillary tube. The sizeof the constrained opening used may be in the range of 10 μm to 2 mm.

The flow impedance to choose for the flow restriction device 150 willdepend on a number of factors, including the pressure used by the highpressure gas source 100, the volumes of the purge and separator tanks 35and 50, the pumping power of the separator tank pressure controller 40,and the maximum tolerable increase in pressure that is to be allowed inthe separator tank 35. Calibration measurements may be used to determinean appropriate flow restriction and/or the flow restriction device 150may be configured to be adjustable so as to respond to differentoperating conditions. Additionally, the flow restriction device may becontrolled by a flow rate controller that provides feedback correctionbased on the pressure in the separator tank 35 (as measured by aseparator tank pressure sensor 32). For example, when it is sensed thatthe pressure in the separator tank 35 has exceeded, or will exceed, athreshold, the controller may be configured to send a signal to the flowrestriction device 150 to stem the flow of gas into the separator tank35 (by increasing its flow impedance). In general, the time to equalizepressures in the separator and purge tanks 35 and 50 will be a majorcontributor to the overall cycle time. Faster equalization may beachieved by providing an additional high-throughput connection to thepurge tank 50 that allows independent and rapid pumping of this volume.

In case the emptying procedure is not well regulated or some othermalfunction occurs, the separator tank 35 may be fitted with a sensor 32to measure the liquid level in the tank. If the liquid level risesbeyond a predefined “maximum fill level” (which will be chosen to besafely below the level of the entry point for the separator tankpressure controller 40), the evacuation system may be configured toenter a safety mode. The safety mode may comprise at least the functionof isolating the separator tank pressure controller 40 from theseparator tank 35 so as to prevent damage to the separator tank pressurecontroller 40.

In general, the separator tank 35 may be arranged to have a volume ofbetween 1 and 10 litres with the purge tank 50 being arranged to be muchlarger. For a separator tank 35 much smaller than 1 litre, it may bedifficult to maintain a stable pressure and emptying would have to becarried out more frequently. On the other hand, a separator tank 35 muchlarger than 10 litres may be considered too cumbersome and may place toogreat a load on the separator tank pressure controller 40.

The control of the flow rate of liquid and gas into the separator tank35 is also affected by the size of the separator tank 35 (and thereforeany residual or buffer volume in the separator tank 35). A large buffervolume means that a greater quantity of gas will need to be provided orremoved by the separator tank pressure controller 40 in order for it tomanipulate the pressure in the buffer volume and thus the flow rate,making it more difficult for the separator tank pressure controller 40to carry out its function.

As a simpler alternative to the embodiment depicted in FIG. 6, theliquid level in the separator tank 35 may be controlled without a purgetank by pumping directly on the liquid in the separator tank 35 via aliquid or two-phase compatible pump 62 connected to a suitable valve inthe lower region of the separator tank 35, as depicted in FIG. 7.According to this arrangement, however, steps may need to be taken toensure that pressure fluctuations in the pump 62 do not disturb thequality of the low pressure state maintained in the separator tank 35 bythe separator tank pressure controller 40.

Due to the expense and space required for multiple vacuum sources, it isoften convenient where a number of different apparatuses require suchsources to provide a shared vacuum facility (a “house vacuum”). In theembodiments discussed above with reference to FIGS. 6 and 7, forexample, a share vacuum facility may be used as the basis for theseparator tank pressure controller 40. The use of a single vacuumfacility for several apparatuses has a number of advantages in terms ofspace-saving and costs, but it may also suffer one or more drawbacks.For example, it may often be necessary to provide an effective andreliable gas-liquid separation system, since liquid will not normally betolerated in the shared vacuum facility. First, because the sharedvacuum facility provides low pressure to various different users in theplant, some of these users may require a certain minimum in terms ofstability of the vacuum. Liquid in the shared vacuum facility willessentially form a 2-phase flow, which is typically unstable. Thus, theingress of liquid into the shared vacuum facility may negatively affectother users. Additionally or alternatively, it is likely that the sharedvacuum facility will not have been designed for moisture. There may beno precautions against corrosion, electrical shorts etc. Consequently,the shared vacuum facility may forbid users from releasing moisture intothe shared vacuum facility.

A gas-liquid separation system may be complex, bulky, prone to seriousfailure with unacceptable consequences and relatively expensive.Furthermore, the performance of such a system may be generallyunsuitable for the present application. For example, the liquid/gasmixture could in principle be extracted by direct connection to aliquid-ring pump without any other hardware in between. The performanceof such a system may be inferior to one or more embodiments of thepresent invention in terms of stability, pulsations, etc., although itmay be possible to extract most of the liquid in this way.

Furthermore, reliance on a shared vacuum facility makes this component asingle-point-failure for all of the apparatuses dependent on it: asingle failure in the shared vacuum facility may lead to failure and/ordamage in a large number of separate apparatuses.

According to one or more embodiments of the present invention, anevacuation system 30 is provided that comprises a two-phase compatiblepump (e.g. a liquid ring pump, or a liquid jet pump) as the main drivingforce to extract the gas/liquid mixture, in combination with the sharedvacuum facility as a backup in case the two-phase compatible pump failsand apparatus damage may be caused due to leakage of the liquid from theliquid confinement structure 12. According to this embodiment,therefore, failure of the shared vacuum facility may not necessarilylead to failure of a large number of apparatuses and no gas/liquidseparation mechanism may be required.

FIG. 8 shows an embodiment comprising a two-phase compatible pump 200 incombination with a shared vacuum facility 210. The gas/liquid mixture isdrawn via hose 14 as before and enters a holding tank 190 through aporous block 195 located at the bottom of the tank. Pore sizes in themicron range are generally well suited to the application in handalthough in extreme cases it may be possible to include pore sizes up toseveral hundred microns. The porous block may be formed from a sinteredmaterial designed, for example, for particle filtering. Materials suchas stainless steel 316L, Hastelloy C-22, or nickel may be used, nickelbeing well suited for dealing with ultra-pure water. The types ofmaterial likely to be suitable are those that may also be used as a flowrestrictor for gases and/or liquids or as a “gas diffuser”, which may beused to reduce the velocity of a purge gas to ensure uniform and laminarflow. Alternatively, the porous block 195 may be formed usingelectrochemical processes, or may be formed using non-metallicmaterials.

The porous block 195 acts to homogenize the mixture of gas and liquid bycreating a uniform suspension of fine gas bubbles in the liquid. Theuniform mixture thus obtained has more constant and predictable flowcharacteristics and can be more easily dealt with by the two-phasecompatible pump 200, which extracts the majority of the liquid/gasmixture via a main pumping line 165. If the mixture were not uniform,slug flow would result in the outflow of the holding tank 190 leading toerratic flow and, therefore, unstable pressure.

The shared vacuum facility 210 provides a backup pumping capability viabackup line 155. As shown in FIG. 8, the outlet for the backup line 155should be towards an upper surface of the holding tank 190 so that,should the main pumping line 165 fail, the evacuation system 30 willcontinue to operate via the backup line 155. In this scenario, theliquid part of the gas/liquid mixture would no longer be pumped out ofthe holding tank 190 but would rather start to settle at the bottom ofthe tank. The backup capacity of the tank 190 should be such as to allowall the liquid confined by the liquid confinement structure to beremoved following a pump failure in order to prevent leakage (the liquidsupply system would typically be shut off).

The shared vacuum facility 210 may be protected from liquid through theuse of a hydrophobic filter 170 on the backup line 155. The hydrophobicfilter acts to allow gas (even humid gas) to pass but blocks the flow ofliquid. A strained Teflon membrane may be suitable for this purpose, adevice of this type being available in the field for use as a particlefilter. The basic structure of this membrane is that of an intertwinednetwork of Teflon strands, resembling “spaghetti”. The use of thismembrane as a hydrophobic filter is based on a side-effect wherebyliquid causes the Teflon strands to swell up and block the gas/liquidflow, whereas pure gas flows (including humid gas) are allowed to pass.However, any other material having the property of blocking the flow ofliquid but allowing gas to pass could also be used. An advantage of thistype of filter is that it is simple and reliable in comparison toalternative mechanisms that serve the same purpose (for example, adevice configured to detect droplets in a gas stream and close a valvewhen droplets are detected, would require a considerably more complexarrangement that is likely to be more expensive to implement and mayhave inferior reliability).

During normal operation, the main pumping line 165 should be configuredto provide a deeper vacuum than the backup line 155 so that the majorityof the gas/liquid mixture passes through the main pumping line 165. Acheck valve 105 may be provided on each line to ensure that they do notdraw on each other. In addition, a backpressure regulator 175 may beprovided on each line to avoid unnecessarily burdening the shared vacuumfacility 210 with work when it is not actually needed. The backpressureregulator 175 in backup line 155 will be set at a lower vacuum (i.e. ahigher pressure level) than provided by the shared vacuum facility 210.Thus, in normal operation, the shared vacuum facility 210 will not drawon the system. Only when the pump 200 fails and the pressure in theholding tank 190 starts to rise, does the backpressure regulator open upand allow the shared vacuum facility 210 to draw on the holding tank190.

FIG. 9 shows an evacuation system 30 with a separator tank 35 in asimilar configuration to FIG. 7. According to the arrangement shown inFIG. 9, however, the separator tank pressure controller 40 comprises amain pumping line 165 and a backup line 155. The main pumping line 165is configured to provide a deeper vacuum than the backup line 155 and ahydrophobic filter 170, a check valve 105 and a back pressure regulator175 may be provided in each line to perform the functions as discussedabove in relation to FIG. 8. As for the embodiment in FIG. 7, theseparator tank 35 can be emptied using a liquid pump 200. A check valve105 may be provided on the separator tank pumping line 63 to ensure thatno backflow occurs from the liquid sink 90.

FIG. 10 shows an embodiment comprising two tanks—a separator tank 35 anda purge tank 50—in a similar configuration to the embodiment describedabove with reference to FIG. 6. Here again, however, the separator tankpressure controller 40 comprises a main pumping line 165 and a backupline 155, with the main pumping line 165 being configured to provide adeeper vacuum than the backup line 155. In addition, check valves 105have replaced the valves 70 and 95 of FIG. 6. This configuration mayreduce the need for system control (either manual or automatic) and mayimprove system reliability by reducing the possibility of error. Thecheck valve 105 located between the tanks 35 and 50 allows liquid toflow from the separator tank 35 into the purge tank 50 under normaloperation but closes off during a purge tank 50 emptying procedure,during which the pressure in the purge tank 50 may temporarily riseabove that in the separator tank 35. The check valve 105 between thepurge tank 50 and the liquid sink 90 ensures that a vacuum can arise inthe purge tank 50 without drawing material back from the liquid sink 90.Again, a limited flow connection 145 comprising a flow restrictiondevice 150 can be used to effect pressure equalization between the twotanks 35 and 50 in a gradual manner so as not to disrupt the pressuremaintained in the separator tank 35.

The embodiment shown in FIG. 10 and analogous embodiments involvingcheck valves rather than active valves, which ensure that flow occursonly in the desired direction without the need for a control system toapply timing valve actuation, etc., may be operated in a continuous “onestate” mode, without, for example, having to switch periodically from anormal operating state to a “purge tank emptying state”. In theembodiment shown in FIG. 10, this is achievable by controlling the rateof flow to and from the purge and separator tanks so that neitherbecomes overly full. An advantage of this type of arrangement may beincreased reliability as no valves will need to switch regularly and notiming actuation will need to take place. If it is desired to increasethe rate at which liquid is drained from the purge tank 50, an upperregion of the purge tank 50 may be connected to a high pressure gassource 100 via valve 101 as described above in relation to theembodiment depicted in FIG. 6.

FIG. 11 shows an alternative arrangement to that shown in FIG. 10,wherein the check valves 105 have been replaced by active valves, whichcan be opened or closed for flow in either or both directions(automatically or manually). The arrangement corresponds closely to thatshown in FIG. 6 except that the separator tank pressure controller 40has been expanded to show a specific configuration, including a mainpumping line 165 backed up by a backup line 155 in a similar manner tothose embodiments discussed above that include main pumping and backuplines 165 and 155.

FIG. 12 shows an alternative to the arrangement shown in FIG. 10, inwhich the high pressure gas source 100 is connected to the purge tank 50via a same valve as the limited flow connection 145, which in turn isconnected directly to the line feeding the main pumping and backup lines165 and 155. This arrangement operates in an analogous manner to theembodiment described above with reference to FIG. 10, but may beimplemented with fewer components and fewer connections to the purgeand/or separator tanks. Valves 101 can be actively actuated and used tocontrol the emptying sequence of the purge tank 50. Actuation of valves101 can be effected electrically or pneumatically, for example.

FIG. 13 depicts an embodiment of the invention that is analogous to thatdiscussed above in relation to FIG. 7, the embodiment differing in thechoice of pump used to drain the separator tank 35. According to thisembodiment, the liquid pump 62 comprises a wheel 110 with one or morecavities 115 connecting with the circumferential surface of the wheel110 so as to provide one or more circumferential openings. The wheeloperates to remove liquid from the separator tank as follows. For agiven cavity 115 and associated circumferential opening, three operatingpositions (or regimes) exist, each corresponding to different angularpositions of the wheel 110 (or ranges of angular position): a liquidfilling position 126, a liquid purging position 127 and a gas purgingposition 128. When the wheel rotates so that a particular cavity 115 isin the liquid filling position 126, that cavity is connected to anopening in the separator tank 35 and liquid from the tank flowsdownwards under gravity into the cavity 115 until the cavity is full.The flow of liquid downwards may tend to reduce the pressure in theseparator tank 35, but if the volume of the cavities 115 are smallenough and/or the separator tank pressure controller 40 is configured torespond quickly enough (by adjusting its pumping power) then thepressure fluctuation in the separator tank 35 may be kept withinacceptable limits. The wheel 110 will eventually rotate the cavity 115away from the filling position 126, the cavity becoming temporarilysealed against a wheel housing 129 when the trailing edge of thecircumferential opening for the cavity 115 moves past the opening in theseparator tank 35. The cavity 115 may remain sealed until the wheelrotation brings the cavity 115 into the liquid purging position 127,wherein the cavity is connected to a liquid sink 90. Most of the liquidstored in the cavity will leave at this point to be replaced by gas(this may be provided by a separate gas supply or be taken from agas-filled volume within the liquid sink). The cavity 115 is thenrotated away from the liquid purging position 127, again becomingtemporarily sealed against the wheel housing 129 before arriving at thegas purging position 128. At this position, the cavity 115 is pumpeddown by a two-phase compatible pump (which in the embodiment shown isachieved via a connection to the main pumping line 165) to prepare forthe influx liquid that will occur when wheel 110 is made to complete the360° rotation back to the liquid filling position 126.

The wheel 110 may be configured to rotate in a discontinuous manner soas to wait for a predetermined period, for example, at one or more ofthe three positions to give sufficient time for the process concerned tobe carried out. Alternatively, the wheel 110 may be rotated continuouslywith the angular velocity chosen as a function of the width of theseparator tank, liquid sink, pump and cavity openings so that enoughtime is available for the purging and/or filling operations to becarried out efficiently. Other effects such as turbulence in theseparator tank 35 and/or undesirably large pressure fluctuations mayoccur for excessive angular velocities. A sliding seal may be used toseal the cavity at intermediate positions 131. Although the embodimentshown has only a single cavity, the wheel 110 may also operate with aplurality of cavities, so that at any given time different cavities maybe exposed to two or more of the three operating positions 126, 127 and128. This type of liquid pump has an advantage that it may be usedcontinuously, which minimizes the risk of pressure fluctuations duringan emptying phase (such as may be the case where a purge tank is used).The simplicity of the mechanism may provide high reliability and,because it is relatively insensitive to variations in the flowproperties of the liquid, it is unlikely to suffer from the pressurefluctuations to which a traditional liquid pump may be prone.

FIG. 14 depicts an alternative pump 62 in a configuration analogous tothat depicted in FIGS. 7 and 13. Here, a gas jet pump is provided toremove liquid from the separator tank 35. A stream of high velocity gas(a “gas jet”) is forced to flow along duct 102, from left to right inthe Figure (arrows 106), by a high pressure gas source 100 (which may bethe same as that used to supply the gas seal, for example). An outletfrom the separator tank 35 is connected substantially at right angles toa venturi nozzle 103, through which the gas jet flows. The venturinozzle 103 constricts the flow, leading to an increase in particle speedand a corresponding reduction in pressure. It is this reduction inpressure that provides the low pressure to extract the liquid from theseparator tank 35. Again, this arrangement may work continuously and canbe arranged to provide a highly controllable pumping power. Due to therelatively simple construction and the fact that it can be operatedusing features (the high pressure gas source 100) that are alreadyprovided in a lithographic apparatus, this liquid pump designs offers apossibly cost effective and space-saving solution. The lack of movingparts means that it may be made to be extremely reliable.

An analogous pump to the gas jet pump can be realized using a liquid jetinstead of the gas jet. This may be preferred as the underlyingmechanism for this type of pump is momentum transfer, and a liquid, suchas water, is likely better at that than gas due to its higher specificmass. Moreover, it may be much more robust against pressure fluctuationswhich are a phenomenon in two-phase flows.

FIG. 15 depicts a further embodiment based on the separator tankconcept. In the arrangement shown, a single two-phase compatible pump200 is used both to extract liquid from the lower portion of theseparator tank 35 and to provide the main pumping power for the lowpressure region in the separator tank 35. As before, a backup line 155is provided, which may be linked to a standard gas pumping system suchas a shared vacuum facility 210. A check valve 105 is located as shownbetween the main pumping line 165 and the backup line 155 to ensure thatthe shared vacuum facility 210 does not pump any liquid from theseparator tank 35. An advantage of this arrangement is reduction in thenumber of components to implement the separator tank concept by using asingle pump to pump out both the liquid and gaseous regions of theseparator tank 35. A flow restriction 107 may be provided that, for agiven flow-rate, maintains the pressure inside the separator tank 35 ata fixed offset relative to ambient pressure. This serves as analternative to a pressure regulator—if or when the flow 14 pulsates, theflow through the flow restriction 107 will compensate the pressurepulsations in the separator tank 35.

FIG. 16 is an alternative embodiment to that described above withreference to FIG. 15, differing in that the single pump 200 used to pumpdown the gaseous region of the separator tank 35 and to remove theliquid therefrom is arranged only to remove liquid. Pressure stabilityin the tank is provided by pressure regulator 108 and gas that has beenseparated from the liquid is removed via line 155. The pressureregulator 108 maintains the pressure in the separator tank 35 at aconstant level by adding a pressure dependent flow, in effect acting tocompensate for flow fluctuations in the line 155. This arrangement mayprovide an evacuation system with a smaller number of components, thuspossibly providing increased reliability and cost effectiveness.

The concept of a main pumping line 165, capable of dealing with liquidand gas, working in combination with a backup line 155, based on gasextraction only, may also be implemented in an evacuation system 30 thatdoes not comprise any separator tank 35 or holding tank 50. Instead a“straight-through” pump concept may be employed as is depicted in FIG.17. Pressure stability/regulation is taken care of according to thisembodiment through the use of a two-phase-compatible pressure regulator300.

In an embodiment, there is provided a lithographic apparatus comprising:an illuminator configured to condition a radiation beam; a supportconstructed to hold a patterning device, the patterning deviceconfigured to impart the radiation beam with a pattern in itscross-section to form a patterned radiation beam; a substrate tableconstructed to hold a substrate; a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate; a liquid supply system configured to at least partly fill aspace between the projection system and the substrate with a liquid, theliquid supply system comprising a liquid confinement structureconfigured to at least partly confine the liquid within the space; anoutlet configured to remove a mixture of liquid and gas passing througha gap between the liquid confinement structure and the substrate; and anevacuation system configured to draw the mixture through the outlet, theevacuation system comprising a separator tank arranged to separateliquid from gas in the mixture and a separator tank pressure controller,connected to a non-liquid-filled region of the separator tank,configured to maintain a stable pressure within the non-liquid-filledregion.

In an embodiment, the evacuation system further comprises: a purge tank,situated lower than the separator tank and connected thereto via anopening in a lower portion of the separator tank and an opening in anupper portion of the purge tank, the connection being controllable via apurge valve; and a pressure equalization connection, controllable by apressure equalization valve, configured to connect respective upperportions of the separator and purge tanks to facilitate flow ofseparated liquid from the separator tank to the purge tank without achange in pressure in the separator tank, wherein the purge tank isconfigured so that liquid can be removed therefrom, during a liquidremoval phase, by closing the purge valve, closing the pressureequalization valve and opening the purge tank to a liquid sink via aliquid sink valve. In an embodiment, the evacuation system furthercomprises a high pressure gas source connectable to the purge tank andconfigured to force liquid from the purge tank to the liquid sink at anincreased rate. In an embodiment, the evacuation system furthercomprises a liquid pump configured to pump liquid from the separatortank to the liquid sink, the liquid pump comprising a gas jet pumppowered by the high pressure gas source. In an embodiment, the purgevalve, the liquid sink valve, or both comprises a check valve configuredto prevent backflow. In an embodiment, the evacuation system furthercomprises: a purge tank, situated lower than the separator tank andconnected thereto via an opening in a lower portion of the separatortank and an opening in an upper portion of the purge tank, theconnection being controllable via a purge valve; and a limited flowconnection configured to connect respective upper portions of theseparator and purge tanks and comprising a flow restriction devicearranged to provide a flow impedance, wherein the purge tank isconfigured so that liquid can be removed therefrom, during a liquidremoval phase, by closing the purge valve and opening the purge tank toa liquid sink via a liquid sink valve, and the flow impedance isselected to equalize the pressures of the separator and purge tanks,after the liquid removal phase, without causing a fluctuation of thepressure in the separator tank that exceeds a threshold value. In anembodiment, the flow impedance is also selected to allow liquid to flowfrom the separator tank to the purge tank at a rate above a minimumtransfer rate. In an embodiment, the evacuation system further comprisesa high pressure gas source connectable to the purge tank and configuredto force liquid from the purge tank to the liquid sink at an increasedrate. In an embodiment, the purge valve, the liquid sink valve, or bothcomprises a check valve configured to prevent backflow. In anembodiment, the evacuation system further comprises: a purge tank,situated lower than the separator tank and connected thereto via anopening in a lower portion of the separator tank and an opening in anupper portion of the purge tank, the connection being controllable via apurge valve; a limited flow connection configured to connect respectiveupper portions of the separator and purge tanks and comprising a flowrestriction device arranged to provide a flow impedance; and a pressureequalization connection, controllable by a pressure equalization valve,configured to connect respective upper portions of the separator andpurge tanks to facilitate flow of separated liquid from the separatortank to the purge tank without a change in pressure in the separatortank, wherein the purge tank is configured so that liquid can be removedtherefrom, during a liquid removal phase, by closing the purge valve andopening the purge tank to a liquid sink via a liquid sink valve, and theflow impedance is selected to equalize the pressures of the separatorand purge tanks, after the liquid removal phase, without causing afluctuation of the pressure in the separator tank that exceeds athreshold value. In an embodiment, the evacuation system furthercomprises a high pressure gas source connectable to the purge tank andconfigured to force liquid from the purge tank to the liquid sink at anincreased rate. In an embodiment, the purge valve, the liquid sinkvalve, or both comprises a check valve configured to prevent backflow.In an embodiment, the evacuation system further comprises a liquid pumpconfigured to pump liquid from the separator tank to the liquid sink. Inan embodiment, the separator tank pressure controller comprises: a mainpumping line connected to a two-phase compatible pump configured to pumpa the mixture; and a backup line connectable to a shared vacuum facilityconfigured to pump gas only, wherein the main pumping and backup linesare connected to the separator tank, and the two-phase compatible pumpbeing configurable to provide a deeper vacuum than the shared vacuumfacility. In an embodiment, the main pumping line is connected to alower, predominantly liquid filled portion of the separator tank, andthe backup line is connected to an upper, predominantly gas filledportion of the separator tank. In an embodiment, the main and backuppumping lines are each fitted with check valves to prevent one pumpingline from drawing on the other pumping line. In an embodiment, thebackup line comprises a hydrophobic filter configured to prevent liquidfrom reaching the shared vacuum facility. In an embodiment, the mainpumping line, the backup line, or both, comprise a back-pressureregulator configured to control the pumping power provided by thetwo-phase compatible pump, the shared vacuum facility, or both, as afunction of the pressure on a separator tank side of the back-pressureregulator. In an embodiment, the two-phase compatible pump is configuredboth to maintain a vacuum level in a non-liquid-filled portion of theseparator tank via the main pumping line and to extract liquid from alower, predominantly liquid filled portion of the separator tank via alower separator tank pumping line, wherein a check valve is positionedbetween the main pumping line and the lower separator tank pumping lineso as to prevent the shared vacuum facility from pumping on the liquid.

In an embodiment, there is provided an apparatus, comprising: apressurized gas input configured to provide gas under pressure to aninterface region of a container from which liquid may escape; astabilized evacuation system configured to provide controlled removal ofa mixture of liquid and gas from the region, the flow of gas caused bythe pressurized gas input coupled with the stabilized evacuation systembeing configured to limit the escape of liquid from the containerthrough the interface region, the stabilized evacuation systemcomprising a separator tank arranged to separate liquid from gas in themixture and a separator tank pressure controller, connected to anon-liquid-filled region of the separator tank, configured to maintain astable pressure within the non-liquid-filled region.

In an embodiment, there is provided a lithographic apparatus,comprising: an illuminator configured to condition a radiation beam; asupport constructed to hold a patterning device, the patterning deviceconfigured to impart the radiation beam with a pattern in itscross-section to form a patterned radiation beam; a substrate tableconstructed to hold a substrate; a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate; a liquid supply system configured to at least partly fill aspace between the projection system and the substrate with a liquid, theliquid supply system comprising a liquid confinement structureconfigured to at least partly confine the liquid within the space; anoutlet configured to remove a mixture of liquid and gas passing througha gap between the liquid confinement structure and the substrate; and anevacuation system configured to draw the mixture through the outlet, theevacuation system comprising a two-phase compatible pump and aliquid/gas homogenizer arranged between the gap and the two-phasecompatible pump, the liquid/gas homogenizer being arranged to provide auniform mixture of liquid and gas to the two-phase compatible pump.

In an embodiment, the liquid/gas homogenizer comprises a holding tankand a porous block, the porous block configured to homogenize themixture as it is passed into the holding tank.

In an embodiment, there is provided a lithographic apparatus,comprising: an illuminator configured to condition a radiation beam; asupport constructed to hold a patterning device, the patterning deviceconfigured to impart the radiation beam with a pattern in itscross-section to form a patterned radiation beam; a substrate tableconstructed to hold a substrate; a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate; a liquid supply system configured to at least partly fill aspace between the projection system and the substrate with a liquid, theliquid supply system comprising a liquid confinement structureconfigured to at least partly confine the liquid within the space; anoutlet configured to remove a mixture of liquid and gas passing througha gap between the liquid confinement structure and the substrate; and anevacuation system configured to draw the mixture through the outlet, theevacuation system, comprising: a main pumping line connected to atwo-phase compatible pump configured to pump the mixture, a backup lineconnectable to a shared vacuum facility configured to pump gas only andarranged to backup the two-phase compatible pump, the two-phasecompatible pump being configurable to provide a deeper vacuum than theshared vacuum facility, and a two-phase-compatible pressure regulatorconnected to the main pumping line and the backup line.

In an embodiment, the backup line comprises a hydrophobic filterconfigured to prevent liquid from reaching the shared vacuum facility.

In an embodiment, there is provided a device manufacturing method,comprising: providing a liquid to a space between a projection system ofa lithographic apparatus and a substrate, the liquid being confined tothe space at least in part by a liquid confinement structure; removing amixture of liquid and gas passing through a gap between the liquidconfinement structure and the substrate; separating, in a separatortank, liquid from gas in the mixture; pumping on a non-liquid-filledregion of the separator tank so as to maintain a stable pressure withinthe separator tank; and projecting a patterned beam of radiation, usingthe projection system, through the liquid onto the substrate.

In an embodiment, there is provided a device manufacturing method,comprising: providing a flow of gas under pressure to an interfaceregion of a container from which liquid may escape; controlled removingof a mixture of liquid and gas from the region, the flow of gas coupledwith the controlled removal of the mixture being configured to limit theescape of liquid from the container through the interface region;separating, in a tank, liquid from gas in the mixture; and pumping on anon-liquid-filled region of the tank so as to maintain a stable pressurewithin the non-liquid-filled region.

In an embodiment, there is provided a device manufacturing method,comprising: providing a liquid to a space between a projection system ofa lithographic apparatus and a substrate, the liquid being confined tothe space at least in part by a liquid confinement structure; removing amixture of liquid and gas passing through a gap between the liquidconfinement structure and the substrate using a two-phase compatiblepump; homogenizing the mixture before it reaches the two-phasecompatible pump; and projecting a patterned beam of radiation, using theprojection system, through the liquid onto the substrate.

In an embodiment, there is provided a device manufacturing method,comprising: providing a liquid to a space between a projection system ofa lithographic apparatus and a substrate, the liquid being confined tothe space at least in part by a liquid confinement structure; removing amixture of liquid and gas, the mixture passing through a gap between theliquid confinement structure and the substrate, through a main pumpingline using a two-phase compatible pump; removing gas from the mixturethrough a backup line using a shared vacuum facility as a backup to thetwo-phase compatible pump, the two-phase compatible pump providing adeeper vacuum than the shared vacuum facility; regulating the mainpumping line and the backup line using a two-phase-compatible pressureregulator; and projecting a patterned beam of radiation, using theprojection system, through the liquid onto the substrate.

In European Patent Application No. 03257072.3, the idea of a twin ordual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two tables for supporting a substrate.Leveling measurements are carried out with a table at a first position,without immersion liquid, and exposure is carried out with a table at asecond position, where immersion liquid is present. Alternatively, theapparatus has only one table.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

One or more embodiments of the present invention may be applied to anyimmersion lithography apparatus, in particular, but not exclusively, tothose types mentioned above. A liquid supply system as contemplatedherein should be broadly construed. In certain embodiments, it may be amechanism or combination of structures that provides a liquid to a spacebetween the projection system and the substrate and/or substrate table.It may comprise a combination of one or more structures, one or moreliquid inlets, one or more gas inlets, one or more gas outlets, and/orone or more liquid outlets that provide liquid to the space. In anembodiment, a surface of the space may be a portion of the substrateand/or substrate table, or a surface of the space may completely cover asurface of the substrate and/or substrate table, or the space mayenvelop the substrate and/or substrate table. The liquid supply systemmay optionally further include one or more elements to control theposition, quantity, quality, shape, flow rate or any other features ofthe liquid.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus comprising: an illuminator configured tocondition a radiation beam; a support constructed to hold a patterningdevice, the patterning device configured to impart the radiation beamwith a pattern in its cross-section to form a patterned radiation beam;a substrate table constructed to hold a substrate; a projection systemconfigured to project the patterned radiation beam onto a target portionof the substrate; a liquid supply system configured to at least partlyfill a space between the projection system and the substrate with aliquid, the liquid supply system comprising a liquid confinementstructure configured to at least partly confine the liquid within thespace; an outlet configured to remove a mixture of liquid and gaspassing through a gap between the liquid confinement structure and thesubstrate; and an evacuation system configured to draw the mixturethrough the outlet, the evacuation system comprising a separator tankarranged to separate liquid from gas in the mixture and a separator tankpressure controller, connected to a non-liquid-filled region of theseparator tank, configured to maintain a stable pressure within thenon-liquid-filled region.
 2. The lithographic apparatus according toclaim 1, wherein the evacuation system further comprises: a purge tank,situated lower than the separator tank and connected thereto via anopening in a lower portion of the separator tank and an opening in anupper portion of the purge tank, the connection being controllable via apurge valve; and a pressure equalization connection, controllable by apressure equalization valve, configured to connect respective upperportions of the separator and purge tanks to facilitate flow ofseparated liquid from the separator tank to the purge tank without achange in pressure in the separator tank, wherein the purge tank isconfigured so that liquid can be removed therefrom, during a liquidremoval phase, by closing the purge valve, closing the pressureequalization valve and opening the purge tank to a liquid sink via aliquid sink valve.
 3. The lithographic apparatus according to claim 2,wherein the evacuation system further comprises a high pressure gassource connectable to the purge tank and configured to force liquid fromthe purge tank to the liquid sink at an increased rate.
 4. Thelithographic apparatus according to claim 3, wherein the evacuationsystem further comprises a liquid pump configured to pump liquid fromthe separator tank to the liquid sink, the liquid pump comprising a gasjet pump powered by the high pressure gas source.
 5. The lithographicapparatus according to claim 2, wherein the purge valve, the liquid sinkvalve, or both comprises a check valve configured to prevent backflow.6. The lithographic apparatus according to claim 1, wherein theevacuation system further comprises: a purge tank, situated lower thanthe separator tank and connected thereto via an opening in a lowerportion of the separator tank and an opening in an upper portion of thepurge tank, the connection being controllable via a purge valve; and alimited flow connection configured to connect respective upper portionsof the separator and purge tanks and comprising a flow restrictiondevice arranged to provide a flow impedance, wherein the purge tank isconfigured so that liquid can be removed therefrom, during a liquidremoval phase, by closing the purge valve and opening the purge tank toa liquid sink via a liquid sink valve, and the flow impedance isselected to equalize the pressures of the separator and purge tanks,after the liquid removal phase, without causing a fluctuation of thepressure in the separator tank that exceeds a threshold value.
 7. Thelithographic apparatus according to claim 6, wherein the flow impedanceis also selected to allow liquid to flow from the separator tank to thepurge tank at a rate above a minimum transfer rate.
 8. The lithographicapparatus according to claim 6, wherein the evacuation system furthercomprises a high pressure gas source connectable to the purge tank andconfigured to force liquid from the purge tank to the liquid sink at anincreased rate.
 9. The lithographic apparatus according to claim 6,wherein the purge valve, the liquid sink valve, or both comprises acheck valve configured to prevent backflow.
 10. The lithographicapparatus according to claim 1, wherein the evacuation system furthercomprises: a purge tank, situated lower than the separator tank andconnected thereto via an opening in a lower portion of the separatortank and an opening in an upper portion of the purge tank, theconnection being controllable via a purge valve; a limited flowconnection configured to connect respective upper portions of theseparator and purge tanks and comprising a flow restriction devicearranged to provide a flow impedance; and a pressure equalizationconnection, controllable by a pressure equalization valve, configured toconnect respective upper portions of the separator and purge tanks tofacilitate flow of separated liquid from the separator tank to the purgetank without a change in pressure in the separator tank, wherein thepurge tank is configured so that liquid can be removed therefrom, duringa liquid removal phase, by closing the purge valve and opening the purgetank to a liquid sink via a liquid sink valve, and the flow impedance isselected to equalize the pressures of the separator and purge tanks,after the liquid removal phase, without causing a fluctuation of thepressure in the separator tank that exceeds a threshold value.
 11. Thelithographic apparatus according to claim 10, wherein the evacuationsystem further comprises a high pressure gas source connectable to thepurge tank and configured to force liquid from the purge tank to theliquid sink at an increased rate.
 12. The lithographic apparatusaccording to claim 10, wherein the purge valve, the liquid sink valve,or both comprises a check valve configured to prevent backflow.
 13. Thelithographic apparatus according to claim 1, wherein the evacuationsystem further comprises a liquid pump configured to pump liquid fromthe separator tank to the liquid sink.
 14. The lithographic apparatusaccording to claim 1, wherein the separator tank pressure controllercomprises: a main pumping line connected to a two-phase compatible pumpconfigured to pump a the mixture; and a backup line connectable to ashared vacuum facility configured to pump gas only, wherein the mainpumping and backup lines are connected to the separator tank, and thetwo-phase compatible pump being configurable to provide a deeper vacuumthan the shared vacuum facility.
 15. The lithographic apparatusaccording to claim 14, wherein the main pumping line is connected to alower, predominantly liquid filled portion of the separator tank, andthe backup line is connected to an upper, predominantly gas filledportion of the separator tank.
 16. The lithographic apparatus accordingto claim 14, wherein the main and backup pumping lines are each fittedwith check valves to prevent one pumping line from drawing on the otherpumping line.
 17. The lithographic apparatus according to claim 14,wherein the backup line comprises a hydrophobic filter configured toprevent liquid from reaching the shared vacuum facility.
 18. Thelithographic apparatus according to claim 14, wherein the main pumpingline, the backup line, or both, comprise a back-pressure regulatorconfigured to control the pumping power provided by the two-phasecompatible pump, the shared vacuum facility, or both, as a function ofthe pressure on a separator tank side of the back-pressure regulator.19. The lithographic apparatus according to claim 14, wherein thetwo-phase compatible pump is configured both to maintain a vacuum levelin a non-liquid-filled portion of the separator tank via the mainpumping line and to extract liquid from a lower, predominantly liquidfilled portion of the separator tank via a lower separator tank pumpingline, wherein a check valve is positioned between the main pumping lineand the lower separator tank pumping line so as to prevent the sharedvacuum facility from pumping on the liquid.
 20. The lithographicapparatus according to claim 1, wherein the evacuation system comprisesa two-phase compatible pump and a liquid/gas homogenizer arrangedbetween the gap and the two-phase compatible pump, the liquid/gashomogenizer being arranged to provide a uniform mixture of liquid andgas to the two-phase compatible pump.